FIG. 1 illustrates a prior art power supply 10 for producing a regulated output voltage from an AC voltage source 12 containing a plurality of phases 14 which may be of variable voltage and/or variable frequency. A fullwave rectifier 16 comprised of diodes D1-D6 is connected to the outputs 18 of the individual phases 14. A first group D1-D3 of the diodes passes positive pulses outputted from the phases 14 to a positive output terminal 20. A second group D4-D6 of diodes passes negative pulses from the phases 14 to a negative output terminal 22. The pulsating current produced by the diodes D1-D3 is filtered by filter 24 which is comprised of inductor 26 and capacitor 28 in a conventional manner to produce a DC output voltage across terminals 20 and 22 which is substantially ripple free. Resistor 29 represents an electrical load applied to the output with it being understood that the load may also include capacitance/or inductance and be variable.
The output voltage across terminals 20 and 22 is regulated by a voltage regulator 30 which is comprised of a plurality of magnetic amplifiers S1-S6 (saturable reactors), which respectively have first and second terminals which connect a main winding 32 in series between the output 18 of one of the phases 14 and one of the diodes D1-D6, a control winding 34 which conducts control current through each of the magnetic amplifiers S1-S6 which is proportional to the output voltage across terminals 20 and 22 and a control circuit 36. As is known, a magnetic amplifier functions as a variable impedance which switches from a high impedance to a low impedance after elapse of a time interval proportional to the control current and the volt-second product across the magnetic amplifier. Accordingly, as the current increases in the control winding of a magnetic amplifier, the time interval required for the impedance to switch from a high level to a low level increases. The control current 36 comprises a current amplifier which controls the current flowing in the common control winding 34 in direct proportion to the magnitude of the voltage at point 24. The point 24 is connected to the control input of the amplifier in the control circuit. Output voltage regulation across points 20 and 22 is insured as a consequence of any change in voltage producing an opposing change in current in the control winding 34. Thus an increase in output voltage causes the control current in winding 34 to increase which increases the time required to change the impedance of the magnetic amplifier from high to low. This time delay decreases the time out of a possible 180.degree. of the rectified voltage during which the fullwave rectifier will conduct to cause a decrease in output voltage. A drop in output voltage produces the opposite effect. Capacitor 38 shunts harmonics from the control current applied to the magnetic amplifiers S1-S6.
If there is a fault in the control circuit 36, the control current applied to the individual magnetic amplifiers S1-S6 can go to zero to cause the time required for the magnetic amplifiers S1-S6 to drop to a minimum value which will cause the output voltage between terminals 20 and 22 to rise to maximum level that can result in damage to the electrical load 29 coupled thereto. Additionally, if an open circuit exists across the terminals 30 and 34, there will be a decrease in current in the control windings 34 of the individual magnetic amplifiers S1-S6 which can lead to damage of a load coupled to the output terminals 20 and 22 as a consequence of the fullwave rectifier 16 outputting maximum power.
Magnetic amplifiers are also known as saturable reactors or transductors. Publications on magnetic amplifiers are "The Magnetic Amplifier" by J.H. Reyner, 1950, "Transductor and Magnetic Amplifiers" by A.G. Milnes, 1957, and "Magnetic Amplifiers" by H.F. Storm, 1955. Magnetic amplifiers were used widely for high power switching applications until approximately 1970. Thereafter, silicon power devices supplanted many usages of magnetic amplifiers. Recently, developmental work has occurred with magnetic amplifiers for applications involving high frequency. U.S. Pat. Nos. 3,422,341, 3,971,975, 4,212,054 and 4,591,966 disclose reactors in series with diodes for outputting rectified current.